I have the following code which is called by many threads using a thread pool:

Qt Code: 
  1. QByteArray PureImageCache::GetImageFromCache(MapType::Types type, Point pos, int zoom)
  2.  
  3.      {  
  4.  
  5.            lock.lockForRead();
  6.  
  8.  
  9.           if(gtilecache.isEmpty()|gtilecache.isNull())
  10.  
  11.           return ar;
  12.  
  13.     QString dir=gtilecache;
  14.     Mcounter.lock();
  15.     qlonglong id=++ConnCounter;
  16.     Mcounter.unlock();
  17.  
  18.         QString db=dir+"Data.qmdb";
  19.         {
  21.  
  22.             cn = QSqlDatabase::addDatabase("QSQLITE",QString::number(id));
  23.  
  24.             cn.setDatabaseName(db);
  25.             cn.setConnectOptions("QSQLITE_ENABLE_SHARED_CACHE");
  26.             if(cn.open())
  27.             {
  28.                 QSqlQuery query(cn);
  29.                 query.exec(QString("SELECT Tile FROM TilesData WHERE id = (SELECT id FROM Tiles WHERE X=%1 AND Y=%2 AND Zoom=%3 AND Type=%4)").arg(pos.X()).arg(pos.Y()).arg(zoom).arg((int) type));
  30.                 query.next();
  31.                 if(query.isValid())
  32.                 {
  33.                     ar=query.value(0).toByteArray();
  34.                     //qDebug()<<"Got";
  35.                 }else{
  36.                     //qDebug()<<"Miss";
  37.                 }
  38.                 cn.close();
  39.             }
  40.         }
  41.         QSqlDatabase::removeDatabase(QString::number(id));
  42.     lock.unlock();
  43.     return ar;
  44. }
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However, I have met one crash (not every time) right after I call the:

Qt Code: 
  1. cn = QSqlDatabase::addDatabase(“QSQLITE”,QString::number(id));
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function. And the call stack says the error is:

Qt Code: 
  1. Debug Assertion Failed!
  2. File:f:\dd\vctools\crt_bld\self_x86\crt\src\dbgdel.cpp
  3. Line: 52
  4. Expression:_BLOCK_TYPE_IS_VALID(pHead->nBlockUse)
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The cause is within the = operator, it call an atomic assign where inside that atomic assign, there's a delete function cause the crash.
I noticed that there's a shared_null pointer inside the default constructor of QSqlDatabase.
Here is the thing:
The default constructor of QSqlDatabase used a shared_null.
Note that the shared_null is an object indicating an invalid database, not a plain null pointer.

Here is the code: http://code.woboq.org/qt5/qtbase/src...1shared_nullEv
Could it be that the delete is called on to delete the shared_null which is essentially some static variable and caused the crash?
How could I avoid this and continue using QSqlDataBase safely in multi-thread senario?